Signaling by frequency modulation



Jan. 20, 1931. H. 0. PETERSON SIGNALING BY FREQUENCY MODUL ATION FiledJuly 12; 1927 UPfRA TING RANGE INVENTOR mow o. mensou 2Z RNEY was 20,193

UNITED STATES PATENT ornea mm 0. PETERSON, OF NIVERHEAD, NEW YORK,ASSIGNOB TO RADIO CORPORATION OF AMERICA, A CORPORATION 01 DELAWARESIGNALING BY FREQUEN MODULATION 2 r Application filed .I'uly 1B, 1927.Serial No. 205,071

This invention relates to frequency (modulation and includes a methodand means for transmitting and receiving a signal wave over a highfrequency carrier wave by 5 slightly changing the frequency and'not theamplitude of the carrier wave in accordance wit the signal wave. 1

The increasing trafiic of all kmds employing various portions. of theavailable.

requency spectrum for the transmission of intelligence makes itdesirable that the necessary wave band re uired for any one channel begreatly re uced. When using amplitude modulation the wave band requiredfor speech telephony extends .over

twice the width of the useful range of audible frequencies, owing to theresulting side 25 object of my invention.

I accomplish the aforesaid objects by slightly changing the frequency ofthe carrier wave at'a frequency dependent on the frequency of the si alwave and to an exnal wave, but without appreciably changing the amlitude of the carrier wave at any time. y this frequency modulation,instead of amplitude modulation, I inherently introduce a frequencwobble which helgs eliminate fading, an avoid the use-of si e bandfrequencies, in the ordinary sense, with their attendent disadvantages.

To change frequency modulation to amplitude modulation it has beensuggested that the receiver be detuned. Forresponse to'small fre uencyvariation I first heterodyne the received energy to obtainfrequencymodulated energy of intermediate frequency, and then analyze theintermediate freguency modulated energy by means of a etunedlcircuit toobtain the. signal wave. 4 The requisite maximum wave band may be madeas small as desired, since the relative frequency change, for thepurpose-of analy tent dependent on t e amplitude of the Sig-- sis, isdependent u on the magnitude of the frequency selecte .as theintermediate frequency. By this means a ran' 'e of only five hundredcycles per second, lnsteadof twenty thousand cycles or more, min thecase of amplitude modulation, suflices for successful operation, even onvery short wave lengths.

The invention is described more in detail in the following specificationfwhich is accompanied by a drawing in which,

Figure 1 indicates a\transmitter for frequency modulation;

Figure 2 indicates an alternative form of variable reactance forobtaining frequency change in response to a signal wave;

F1gure3 represents a heterodyne receiver capable of analyzing andtranslating frequency modulated signals; and

Figure 4 is explanatory of the method of reception.

Referring to Figure 1 there is a. microphone circuit or other source ofa signal wave 2, whichmay be coupled to a suitable amplifier 4, and thenconve ed over a wire "line 6 for use in obtaining requency modu-.

lation' in a transmitter. A high frequency carrier wave is generated ina vacuum tube osclllator 8 having input and output inductances 10 and12, and a coupling andtuning condenser 14. The anode circuit is.energlzed from a source of direct current 16, which is by-passed forhigh frequency oscillations by a condenser 18.

In parallel with the tuning oondenserl-l there is a variable condenser20, one plate of which is vibrated by means of an armature 22 which actsunder the influence of an. electro-magnet 24 which is energized by theenergy ofthe signal wave. It is clear that the magnitude of thefrequency variation is determined by the amplitude of movement of thearmature 22, and that the'efi'ective frequency variation for a givenamplitude of movementmay be made very small, being governed by therelative magnitude of. the two parallel connected condensersm and 20,

and that for a fixed magnitude ofthe vari able condenser 20 the meanfrequency of the carrier wave is adjustable by means of the tuningcondenser 14:. v

The frequency modulated output of the oscillator is coupled to a poweramplifier 26, the output from which may be conveyed over high frequencylines, or radiated from a suit able antenna 28.

In the foregoing modification the frequency variation was obtained byvarying a capacitance, but a variable inductance may equally well beused, in accordance with the modification shown in Figure 2. There theoutput a frequency of the oscillator is determined by a resonant circuitcomprising a fixed inductance 42, a tuning condenser 44, and aninductance 46 which is varied by movement of a copper plate 48 locatedin its flux field. This plate is reciprocated about, its pivot 49 bymeans of an armature 50 which responds to variations in the magneticfield of an electromagnet 52, to which the signal wave is applied.

In case telegraphy signals are to be transmitted an alterating currentof audible frequency may be keyed, as is indicated by the key 54controlling the alternator 56. The alternator frequency may besuper-audible, in'

- quency output is fed to an intermediate amplifier 66, which mayinclude several stages of tuned amplification in cascade to accentuatethe resonance curve in order to obtain a steep slope on its side. Thisreceiver apparently resembles an ordinary super-heterodyne receiver, butdiffers in thatit must include some means to analyze the frequencymodulated energy of intermediate frequency- This is most simply done bytuning the in-' termediate frequency amplifier 66 to a frequency whichlies outside of the operating range of the intermediate frequency.

Thus, referring to Figure 4, the intermediate frequency amplifier istuned not to the the mean carrier frequency f but rather to a frequencyf,, which is so different from the mean frequency that all of theoperating range of intermediate frequency lies to one side of theresonance frequency f,. This resultsin an amplitude change in responseto the frequency variation, which is rectified in a suitable detectortube 68, the output from which is translated by translating means 70.

It is to be understood that the input circuit of the detector tube 68may itself be made the analyzing circuit ifla less steep resonance curveis required. It is also to be understood that where the intermediatefrequency amplifier is used as the analyzer the tuning need not be doneby varying the tuning of the am-- atively detuned frequency f I considerthe use of 'a'prelimifnary' hetero-- dynev desirable in all cases, andessential if slight frequency variation is employed, in-

asmuch as it increases the frequency variation relative to the frequency0 the carrier on which it is impressed,thereby making analysis morepracticable. Merely steepenin the resonance curve will not solve the prolem, for besides the difiiculty of constructing exceedingly efficientcircuits commercially, it 7 must be remembered that such circuits willtend to average out rather than follow the rapid frequency variationsimpressed on them.

Having thus described my invention 1 claim:

1. The method of receiving signals sent by frequency modulation whichincludes the step of heterodym'ng the received energy with energy ofconstant frequency to obtain frequency modulated energy of intermediatefrequency in order to increase the degree of frequency modulation of theintermediate fr uency energy relative to the degree of mo ulation of theincoming signal energy whereby analysis anddetection of the signal isfacilitated.

2. The method of receiving signals sent by frequency modulation whichincludes heterodyning the received energy to obtain frequency modulatedenergy of intermediate frequency of augmented modulation relative to thesignals sent by frequency modulation, analyzing and. rectifying theintermediate frequency energy of augmented modulation, and translatingthe rectified energy.

3. The method of receiving signals sent by frequency modulation whichincludes heterodyning the received wave to obtain an inter mediatefrequency modulated wave of relatively augmented modulation, amplifyingthe intermediate frequency wave in a resonant circuit tuned to afrequencywhich lies outside of the operating or modulation ran of theintermediate frequenc rectifying t e amplified energy, and trans atingthe rectified energy.

4. A receiver for frequency modulated signals comprising means toheterodyne the received energy to obtain energy of intermediate frequencof relatively augmented frequency mod ation, an analyzing circuittunedto a frequency outside ofthe operating or modulation range ofintermediate frequencg, and signal translating means.

5; receiver for frequency modulated siginc 1,7aa,ar1

nals comprisinga source of local oscillations of constant frequency anda combining device to heterodyne a received frequency modulated wave toa wave of intermediate frefl y of r latively augmented frequencymodulation, a resonant intermediate frequency amplifier tuned to afrequency outside of the operating or modulation range of intermediatefrequency, means to rectify the m amplified energy, and means totranslate the rectified enemy OLD O; PETERSON;

